Thermal equilibrium polarization: a near-surface effect in dipolar-based molecular crystals

Recently, observed nonlinear optical and pyroelectric effects in channel-type lattices hosting aligned dipolar moelcules have been explained by a Markov model of polarity formation. As a result of selective attachment processes during crystal growth, two adjacent macrodomains are formed which show opposite net polarity. Although a state near thermal equilibrium was assumed for the attachment of dipoles, resulting crystals are metastable with respect to the global free energy minimum of chains coupled only in one dimension. In the present work we demonstrate that the free energy difference of the two ensembles (growth vs. bulk equilibrium), ΔF, can be smaller by two orders of magnitude than realistic energies of activation for a re-orientation of dipoles. When linear chains are subjected to thermal equilibrium, the system develops polarity as well, however, near the surface domains at both ends chains feature opposing net polarity. The scale length of dipolar ordering can be as large as 20 times the molecular length. Stochastic simulations in two dimensions indicate that both scale length and net polarity are sensitive to even small lateral interactions (ΔElateral/ΔEchain<0.01).